The present invention relates to solid state image sensors. More specifically, the present invention relates to a method for fabricating color image sensors and to a color image sensor fabricated by the method.
Solid state color image sensors are used, for example, in video cameras, and are presently realized in a number of forms including charge-coupled devices (CCDs) and CMOS image sensors. These image sensors are based on a two dimensional array of pixels. Each pixel includes color filter located over a sensing element. An array of microlenses located over the color filter focuses light from an optical image through the color filter into the image sensing elements. Each image sensing element is capable of converting a portion of the optical image passed by the color filter into an electronic signal. The electronic signals from all of the image sensing elements are then used to regenerate the optical image on, for example, a video monitor.
FIG. 1(A) is a cross-sectional view showing a portion of a conventional color image sensor 10. Color image sensor 10 if formed on an n-type semiconductor substrate 11 having a p-well layer 15. An array of photodiodes 20 and charge transfer regions 25 are formed in p-well layer 15, and are covered by a silicon oxide or nitride film 30. A polysilicon electrode 35 is located over charge transfer region 25 such that it is surrounded by film 30. A photo-shielding metal layer 40 is formed over electrode 35, and a surface protective coating 45 and a planarization layer 50 are formed over metal layer 40. A color filter layer 60 is formed on planarization layer 50, and an intermediate transparent film 70 is formed over color filter layer 60. A microlens 80 for focusing light beams 85 is formed from silicon dioxide (SiO2) or a resin material on intermediate transparent film 70. An air gap 90 is provided over microlens 80, and a glass packaging substrate 95 is located over air gap 90.
In operation, light beams 85 are focused by microlens 80 through color filter layer 60 such that they converge along the focal axis F of microlens 80 to strike photodiode 20, wherein photoenergy from light beams 85 frees electrons in photodiode 20. When a select voltage is applied to polysilicon electrode 35, these freed electrons generate a current in charge transfer region 25. A sensor circuit (not shown) of color image sensor 10 then determines the amount of light received by photodiode 20 by measuring the amount of current generated in charge transfer region 25.
Conventional solid-state imaging device 10 is designed for light beams 85 whose incident angle is perpendicular to substrate 11, as shown in FIG. 1(A), before being focused by microlens 80 onto photodiode 20. However, during actual operation of color image sensor 10, light beams can strike microlens 80 at oblique incident angles. A consequence of these oblique light beams is shown in FIG. 1(B). In particular, light beams 87 enter microlens 80 at an oblique angle, which directs light beams 87 away from focal axis F such that they converge at the edge of photodiode 20. Because the photoenergy of light beams 87 is not fully transferred to photodiode 20, color image sensor 10 is unable to generate an accurate image.
Another problem associated with conventional solid-state imaging device 10 is that non-standard packaging methods are required due to the formation of microlenses 80 over color filter layer 60 and intermediate transfer layer 70. Standard packaging methods typically include securing a glass substrate to an IC device using a layer of cement (e.g., epoxy). This cement typically has an index of refraction that is the same as silicon-dioxide and other resins typically used to form microlens 80 and other layers of conventional solid-state imaging device 10. Therefore, to facilitate proper focusing of the light beams, air gap 90 must be provided between glass packaging substrate 95 and microlens 80. Because air gap 90 is used in place of cement, the packaging method used to produce conventional solid-state imaging device 10 is non-standard.
It would be possible to avoid the oblique light beam problem (discussed above) by moving microlens 80 closer to photodiode 20, thereby shortening the distance traveled by the light beams between microlens 80 and photodiode 20. This shortened distance would reduce the displacement of focused oblique light beams 87 (see FIG. 1(B)) relative to the center of photodiode 20, thereby transferring more photoenergy from these oblique light beams to photodiode 20.
One possible method of moving microlens 80 closer to photodiode 20 would be to reduce the thickness of the various layers located below microlens 80. A problem with this method is that the thicknesses of these underlying layers are not easily reduced. First, photo-shielding layer 40 is typically formed during the formation of aluminum wiring utilized to transmit signals to and from each pixel of conventional solid-state imaging device 10. Therefore, the thickness of photo-shielding layer 40 is limited by the wiring specifications. Repositioning microlens 80 closer to photodiode 20 is further restricted by planarization layer 50, which is required to provide a flat surface for forming color filter layer 60 and microlens 80. Therefore, it is not possible to significantly reduce the distance between a surface-mounted microlens 80 and photodiode 20 in conventional solid-state imaging device 10 by reducing the thickness of the layers underlying microlens 80.
Another possible method of moving microlens 80 closer to photodiode 20 would be to form microlens 80 under color filter layer 60 (i.e., between photodiode 20 and color filter layer 60). This arrangement would also address the non-standard packaging problem because, with color filter layer 70-located above microlens 80, it would be possible to use cement to secure glass packaging substrate 95 according to standard packaging methods. However, forming microlens 80 under color filter layer 60 is not practical because, as discussed above, the index of refraction of conventional microlens materials (i.e., resin) is the same as that of other materials typically used to produce conventional solid-state imaging device 10. Therefore, because air gap 90 must be provided over conventional microlens 80, it would be very difficult to produce conventional solid-state imaging device 10 with microlens 80 located under color filter layer 60 using conventional microlens materials.
What is needed is a method for fabricating a color image sensor that minimizes the distance between the microlens and photodiode, and minimizes the fabrication and production costs of the color image sensor.
The present invention is directed to a method for producing a color CMOS image sensor in which the microlens structure is embedded (i.e., located between the photodiode array and the color filter layer), thereby avoiding the oblique light beam problem, discussed above, because each microlens is located closer to its associated photodiode than in conventional image sensor structures. In addition, because the color filter layer is located above the microlenses and sandwiched between two color transparent layers, conventional image sensor packaging techniques (i.e., applying cement to the upper color transparent layer, then applying a glass substrate) may be utilized to produce color CMOS image sensors.
In accordance with a first embodiment of the present invention, an image sensor is produced by depositing a dielectric (e.g., silicon-nitride) layer over an image sensing element (e.g., a photodiode), etching the dielectric layer to form a microlens, and then depositing a protective layer on the microlens, wherein the protective layer has an index of refraction that is different from that of the dielectric. When silicon-nitride is utilized as the dielectric, conventional protective layer materials may be formed on the microlens because the refractive index of silicon-nitride is different from silicon-dioxide and other materials utilized as conventional protective layer materials. Therefore, the silicon-nitride microlenses of the present invention may be embedded under conventional protective materials without eliminating the optical performance of the microlenses. In alternative embodiments, other dielectrics may be used to form the microlens, provided the protective materials formed on the microlens have an index of refraction that is different from that of the dielectric. Because the microlens surface is located below a protective layer, conventional packaging techniques may be used that attach the protective layer to a substrate using cement, thereby reducing manufacturing costs and complexity.
In accordance with another embodiment of the present invention, a color image sensor is produced by depositing a silicon-nitride layer over an image sensing element (e.g., a photodiode), etching the silicon-nitride layer to form a microlens, depositing a color transparent layer on the microlens, and then forming a color filter on the color transparent layer. The silicon-nitride microlens has an index of refraction that is different from the color transparent layer, thereby forming an effective microlens structure that is embedded below the color filter. By forming the microlens below the color filter, the microlens is positioned closer to the image sensing element, thereby minimizing the oblique light beam problems, described above. In addition, by forming a second color transparent layer over the color filter, conventional packaging techniques may be used that attach the second color transparent layer to a substrate using cement, thereby reducing manufacturing costs and complexity.
The novel aspects of the present invention will be more fully understood in view of the following description and drawings.